Defect-free asymmetric Matrimid® gas separation membranes using dihydrolevoglucosenone (Cyrene™) as a greener polar aprotic solvent than traditional solvents

Abstract

Industrial membrane manufacturing via nonsolvent-induced phase separation (NIPS) generates large volumes of solvent-contaminated wastewater. An effective solution is to reduce the inherent toxicity of NIPS casting solutions, or dopes, by substituting traditional polar aprotic solvents with potentially benign alternatives such as dihydrolevoglucosenone (Cyrene™). This study demonstrates the reproducible preparation of high-flux defect-free asymmetric Matrimid® gas separation membranes via dry/wet NIPS using Cyrene™ as a majority dope formulation component. Defect-free membrane performance, as benchmarked for pure gas H2/N2 and O2/N2 separations, is maximized with high loadings of Cyrene™ relative to the volatile co-solvent tetrahydrofuran (THF) in casting solutions (volume ratios up to 3.20:1), which exceed the value of 1.68:1 used for Udel® polysulfone in our previous study. Cyrene™'s poor thermodynamic compatibility with the water coagulant and ability to produce highly viscous dopes ensure minimization and/or elimination of sublayer macrovoids while enabling defect-free membrane preparation at low (i.e., 12–14 wt %) Matrimid® and THF concentrations. Defect-free H2 permeance values up to 164 ± 14 GPU were achieved with a combination of high Cyrene™:THF volume ratios, low Matrimid® concentrations, and short dry step times (i.e., <15 s). Membrane performance achieved in this study matches or exceeds other literature reports of defect-free asymmetric Matrimid® membranes prepared with traditional NIPS solvents.